Many security installations include sensors for detecting the breaking of glass which may occur as a result of an unauthorized intrusion into a secure area. These sensors and their associated circuitry generally fall into two classes. First, some sensors are physically attached to the glass that is to be monitored. Examples of such systems are shown in the U.S. Pat. Nos. to Yanagi 4,091,660 and McCluskey 3,863,250. These systems usually involve audio discriminators in the nature of mechanical filters to remove acoustic energy which is not within the spectrum of the sounds made by breaking glass. For example, McClusky utilizes layers of material interposed between the glass and the surface of a piezoelectric element which filter acoustic vibrations which are not characteristic of breaking glass. Since such sensors are mounted directly to the glass to be monitored, they may be defeated if the glass can be cut and the leads connecting the sensor to the remainder of the alarm system can be cut. Also, despite their filtering, such sensors can react to impacts on the glass which do not break it and which are caused by animals, flying debris, or the like.
Other sensing systems utilize broad band microphones which "listen" for the sound of breaking glass. These microphones usually have a wide bandwidth, and electronic filtering is incorporated to discriminate between the sound of breaking glass and other sounds that may be similar. For example, the McCormick et al., U.S. Pat. No. 4,134,109 includes timing and filtering circuits to process the signals from the microphones so that only breaking glass is detected. The theory of the McCormick et al patent is that there will be two sounds separated by a timing interval. If a window or door is to be monitored, the glass will shatter when struck and will shatter again as it hits the floor. The audio discriminator is designed to listen for this sequence of events.
However, not all installations include hard floors where two distinct shattering sounds may be heard. In such cases attempts have been made to provide audio discriminators which tune to the frequency of breaking glass. According to the Yanagi patent, breaking glass has a frequency peak at approximately 50 kilocycles. Many conventional audio discriminator circuits tune for the sound of breaking glass in the 25,000- to 50,000-cycle range. In the past this has led to a high incidence of false alarms because many ordinary environmental or household sounds have frequency components or harmonics in the 25,000- to 50,000-cycle range. For example, the sound made by the jangling of keys on a key chain will usually activate most glass break detector systems of this type. This is unacceptable because of the number of false alarms which can be generated by such ordinary sounds.